In Switzerland, two pioneers are coming closer and closer to a flight around the world powered only by solar energy.

It doesn’t make good business sense, physics sense, or much of any kind of sense, to try to fly an airplane on solar power. Not yet. With the state of the technology, and how relatively young the solar sector still is, such an endeavor would be considered quixotic today—let alone in 2003, when Bertrand Piccard and André Borschberg, co-founders of Solar Impulse, announced they would design a solar-powered aircraft and fly it around the world. It would be a statement, they said, about our global dependence on fossil fuels and the untapped promise of burgeoning green technologies. The Swiss pilot-entrepreneurs were after “perpetual flight”: a plane that could climb to 9,000 feet and fly on the sun’s energy by day, then descend below cloud cover to lower altitudes, where it would cruise on stored battery power by night.